Single Dagger Board vs Double Boards

Discussion in 'Multihulls' started by CatBuilder, Apr 26, 2011.

  1. Alan.M
    Joined: Sep 2005
    Posts: 37
    Likes: 2, Points: 0, Legacy Rep: 27
    Location: Australia

    Alan.M Junior Member

    But as I said before, there is almost always a huge drag assymetery in any sailing boat. DDW (and under a symmetrical kite) would be the only possible situation where this would not be present.

    And in the boats I've sailed on, DDW doesn't require any board - so that argument is pretty moot IMO.

    Also there is the fact that proas and trimarans live with assymetrical loads constantly, yet seem to perform perfectly well.
     
  2. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 43, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    On my olde MacGregor 36 with one board I could tell that leeway prevention was better on one tack than the other, but I was the only one that noticed. The helm really didn't give a damn if the one board was up or down, same same for the guy holding the tillerbar. You might feel a momentary change in helm "feel" when you dropped the board while sailing, but I doubt the helm settled anywhere different with the board up or down, at least not anything measurable.

    Steve


     
  3. CatBuilder

    CatBuilder Previous Member

    Those are a couple of very interesting replies just above this one. Thanks for the input.

    I'm nearly sold on this single board idea. If only I had exact numbers to know what size to make it. Fairly important, as I'm cutting out bulkheads next week for the hull that would have the dagger case in it.

    Too bad my designer is MIA.
     
  4. rattus
    Joined: Feb 2008
    Posts: 61
    Likes: 4, Points: 8, Legacy Rep: 74
    Location: US

    rattus SeƱor Member

    I'd guess that if you kept the aspect ratio the same, and used the same profile, that the lift would be roughly proportional to area, with some fudging for end effects and Reynolds number.

    Induced drag should be somewhat less with a single board vs. two.

    Mike
     
  5. rayaldridge
    Joined: Jun 2006
    Posts: 581
    Likes: 26, Points: 0, Legacy Rep: 322
    Location: USA

    rayaldridge Senior Member

    Shuttleworth did in fact tank test the various configurations at the Wolfson Unit in Southhampton. He then fed the results into a VPP program to arrive at his decision. It would have been impractical to build 70 foot test sleds, so I don't know what else he could have done.

    I just don't know how you can look at all the asymmetric forces acting on a sailboat, and arrive at the conclusion that the very minor amount of skin friction involved in a single board has any significant negative effect.

    But I'm starting to like the idea of a central foil, despite its decreased efficiency. It seems to have a lot of pragmatic advantages, especially for the little cruising cat I'm building.
     
  6. sabahcat
    Joined: Dec 2008
    Posts: 792
    Likes: 28, Points: 0, Legacy Rep: 273
    Location: australia

    sabahcat Senior Member

  7. CatBuilder

    CatBuilder Previous Member

    Uh, yeah. Kurt Hughes 45.
     
  8. sabahcat
    Joined: Dec 2008
    Posts: 792
    Likes: 28, Points: 0, Legacy Rep: 273
    Location: australia

    sabahcat Senior Member

    My opinion?
    Twin boards like in this pic
    Supporting shelving tying it into hull side

    [​IMG]
     
    1 person likes this.
  9. CatBuilder

    CatBuilder Previous Member

    Ok, thanks. That is one of the possibilities. Why do you suggest the twin boards? What are the advantages you see that make it worthwhile to build two boards and trunks?
     
  10. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 43, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    CB, I believe thats a question you're going to have to answer. The pro's and con's are pretty much out there:

    Twin boards:

    pro: better windward performance, redundancy (questionable value)

    con: additional: weight, complexity, cost, maintenence, drag, time to build, loss of space for galley

    Single board:

    Pro: less: weight, complexity, cost, maintenence, time to build, drag, more and better space for galley

    con: reduction of windward performance

    I wonder how the nudist's on Zeevonk fit their galley??

    Steve



    better space from your galley

     
  11. cavalier mk2
    Joined: Mar 2010
    Posts: 2,123
    Likes: 55, Points: 48, Legacy Rep: 214
    Location: Pacific NW North America

    cavalier mk2 Senior Member

    I talked to Ed Horstman once about one board only and he was of the opinion that with enough section area there shouldn't be any noticeable performance difference in a cat not flying a hull. His Tristar trimarans with the ama daggers gave him a lot of time to observe performance of offset foils. If you have shallow keel skegs for beaching they can help you to windward in a pinch without the weight of redudancy. Having the galley opposite the daggerboard hull makes sense to counterbalance the weight as trunks and boards aren't light. 2 daggers might be easier to resale but building your own should reflect your needs/wants.
     
  12. CatBuilder

    CatBuilder Previous Member

    Yeah, I'm really leaning toward the single board, both for extra room in the galley and for simplicity and speed of building the boat.

    Steve (Keys): Zeevonk has the galley up configuration, so no issue. Actually, I suspect Zeevonk was a custom or semi-custom design specifically for those owners that Kurt now sells as a stock plan. That's my guess, anyway.

    Based on all the posts, I'm not sure there is a reduction in windward ability having one board. It would seem there is completely disagreement on that in this thread.

    Thanks for all the input on this thread.

    Do you think resale will be less for a single board vs. two boards? That's a major concern. I would think it might be higher due to ease of use/handling, but am unsure as to what the market thinks about single vs double boards.
     
  13. cavalier mk2
    Joined: Mar 2010
    Posts: 2,123
    Likes: 55, Points: 48, Legacy Rep: 214
    Location: Pacific NW North America

    cavalier mk2 Senior Member

    Symmetrical layouts are easier to sell but you should be able to find a buyer who has reached similar conclusions. A demonstration sail and meal prep in the galley would help enlighten the fence sitters. There are good arguments both ways on the topic.
     
  14. rayaldridge
    Joined: Jun 2006
    Posts: 581
    Likes: 26, Points: 0, Legacy Rep: 322
    Location: USA

    rayaldridge Senior Member

    Shuttleworth seems very concerned with windward performance, makes it a centerpiece of his design philosophy, and yet likes single boards.

    I think its possible the single board would have a negative effect on resale value. I believe that although the Macgregor 36 originally came out with a single board, customers wanted symmetry, and eventually the design was modified, so that later versions had two boards. I wouldn't swear to it, but I seem to remember hearing that story somewhere.
     

  15. oldsailor7
    Joined: May 2008
    Posts: 2,097
    Likes: 40, Points: 48, Legacy Rep: 436
    Location: Sydney Australia

    oldsailor7 Senior Member

    Why don't you phone Roger MacGregor and ask him.
    His phone number is 949 642 6830.
    I found him a very nice and helpful person and I am sure he wouldn't mind. :cool:
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.